The storage of shipment in bulk or scrap materials such as cuttings or turnings containing iron has always been subject to the problem of severe overheating with the threat of spontaneous combustion, and this problem has remained without a practical solution, partly because the relatively low monetary value of such scrap material tends to make treatment uneconomic unless it can be done very inexpensively. The severity of the problem is indicated by a regulation of the U.S. Coast Guard that no ship can sail with a cargo of this type until, after loading, the temperature of the bulk cargo either falls below 150.degree. F, or where the temperature exceeds 150.degree. F, it must show a continuous downward trend for at least 8 hours. Disturbing of the bulk material due to the loading thereof into a hold aggravates the tendency to overheat, and it is a frequent occurrence for a fully loaded vessel to be in port for many days waiting for cargo heating to subside, resulting in heavy financial loss to the operators and high insurance rates.
Several different approaches have been tried to solve the problem. One approach is to deny the cargo air, for instance by introducing a passivating gas such as nitrogen or ammonia or carbon dioxide into the hold, or by conditioning the air in the hold to reduce its moisture content. U.S. Pat. No. 3,704,088 to Nagel et al and U.S. Pat. No. 3,723,059 to Thum et al are of this type.
In U.S. Pat. No. 3,551,215 to Claiborne et al the temperature of the metal particles is raised to a high level and then the particles are treated either with oxygen or steam before cooling them, but this would be uneconomical because of the high energy input required to reach the elevated temperatures involved.
Another approach is to treat the turnings or cuttings with chemicals which inhibit corrosion. In U.S. Pat. No. 2,913,305 to Andersen the turnings or cuttings are corrosion inhibited by treating their surface with organic acid salts which tend to coat them. In U.S. Pat. No. 2,156,357 to Simpson the treatment of sheet metal surfaces is done with calcium oxide or calcium hydroxide which prevents the iron sheets from rusting during shipment, it being well known that these calcium compounds have an anti-corrosive effect on iron.
During a series of early experiments run by the applicant while seeking a solution to the problem, various approaches were tried which were not successful. In one experiment, blocks of dry ice were packed in a hold interspersed with a cargo of iron turnings on the theory that the carbon dioxide from the dry ice would displace the air between the turnings and leave the cargo in an inert atmosphere. However, the approach failed partly because convection due to the overheating brought in enough new air to sustain the corrosion, and partly because the carbon dioxide failed to disperse adequately throughout the cargo leaving hot pockets. Next, the applicant tried treatment with calcium oxide or hydroxide by spraying it into the cargo, or by dry mixing powder with the turnings, but this approach yielded no cure apparently because there were enough surface areas of the turnings which remained uncoated that the overheating of the bulk proceeded with little abatement.
Basically, the problem of overheating is attributable to corrosion of an exothermic nature, the heat generated thereby being spread in the cargo by convection so that the heating effect tends to become a chain reaction. Industrial turnings contain several percent by weight of machining oil as well as combustible debris and substantial amounts of water.
Two degrees of overheating are common when turnings are confined in a ship's hold. In the case of relatively new uncontaminated good quality turnings, the temperature tends to rise to temperatures in the vicinity of 200.degree. F, so that the water vaporizes and is given off as steam. Such turnings tend eventually to cool down again, resulting in no damage to the cargo or the ship. However, where the turnings are old, are of poor quality and/or contaminated with flammable debris, the overheating proceeds beyond the 200.degree. level and tends to reach the vicinity of 340.degree. F whereat the oil is burned off with a heavy white smoke, and if the cargo continues heating toward 500.degree. F it may be engulfed and the ship endangered. Once such elevated temperatures are approached there is little chance of reversing the process. It can not be extinguished using sea water (short of flooding) or using extinguisher chemicals, because these are themselves corrosive and only aggravate the overheating. The chain reaction which depends on oxygen could be stopped if it were possible to cut off the oxygen supply, but this cannot be done in the ordinary case since convection prevents effective dispersion of passivating gases such as nitrogen or carbon dioxide through the cargo. For instance, spraying the top of the cargo with liquid nitrogen produced no noticeable improvement in an overheating situation where it was tried. It is expected that it would be possible to re-design a hold in a ship to permit effective displacement of the oxygen from the cargo by some other gas, but this is not a practical solution in the case of presently existing ships.